Nicotianamine and histidine/proline are, respectively, the most important copper chelators in xylem sap of Brassica carinata under conditions of copper deficiency and excess

The effect of two different copper conditions (deficiency andexcess) on the amino acid composition in B. carinata xylem sapwas analysed. When the Cu in the nutrient solution was increasedfrom 0.12 to 2.5 or 5 µM, the concentrations of histidine,threonine, glutamine, proline, methionine, and glycine weremuch increased in the xylem sap. When Cu was made deficientin the nutrient solution by decreasing its concentration from0.12 µM to 0 µM, nicotianamine, glutamine, and threoninewere significantly increased in the xylem sap. Aqueous solutionscontaining different Cu–amino acid complexes (simulatedsaps) responded in a specific way to the changes in pH, providinga signature that was used to evaluate, by comparison with thereal xylem sap, the importance of each amino acid in the xylemtransport of Cu. For a single amino acid, the free solutionCu²⁺ concentration versus pH titration curves for histidineand proline were the most similar to that for xylem under Cuexcess. Under Cu deficiency, this Cu concentration versus pHtitration curve appeared to be very similar to that for nicotianamine.It is concluded that increased Cu concentrations induced theselective synthesis of certain amino acids in the sap, of whichhistidine and proline are the most important. Under Cu deficiency,the concentration of nicotianamine was induced the most. Thefact that nicotianamine is induced under Cu starvation and notunder Cu excess, is in contrast to similar studies indicatingspecies-specific reactions. However, the induction of nicotianamineunder Cu starvation is in line with recent molecular data ofthe role of nicotianamine in intracellular Cu delivery. Key words: Brassica carinata, copper, histidine, nicotianamine, proline, xylem sap Received 30 September 2008; Revised 16 October 2008 Accepted 20 October 2008